This module provides a less verbose API for accessing the options
of the various descriptor messages types. There are seven
different option types. The EnumValueOptions are not currently
settable in the proto file. To access extension keys the
descendKey functions are provided as the descend functions ignore
them. The toDP, toEP, toSP are type-specific descents that
are demonstrated in the the tests below. They are useful in that
they provide more information for the type-checker. The toFP,
toEVP, and toMP are fully type-specific descents but are
needed to make the tests below type-check, though they could have
been used in test4, test5', and test7.